“Environmental risks like air pollution are not large in magnitude – their importance comes because everyone in the population is exposed. So when you multiply these small risks by lots of people, all of sudden there can be lots of cases. In a large city, it could be a meaningful number, particularly given the fact that these tumours are often fatal.”

They are not the first to note the danger; Carrington writes that "the discovery of abundant toxic nanoparticles from air pollution in human brains was made in 2016. A comprehensive global review earlier in 2019 concluded that air pollution may be damaging every organ and virtually every cell in the human body."

Ambient UFPs may represent a previously unrecognized risk factor for incident brain tumors in adults. Future studies should aim to replicate these results given the high prevalence of UFP exposures in urban areas.

UFPs come from many sources; according to another study, gasoline powered cars generate 40 percent of the UFP in the Eastern US, followed by industrial sources (33 percent) and diesel. Surprisingly, the biggest indoor sources are wax candles; our usual villains, smoking and frying meat, put out bigger particles. They are barely regulated anywhere but should be; a Spanish expert who reviewed the new research tells the Guardian that "this is an important finding, given that UFPs are directly emitted by combustion cars and several studies in animals have shown UFPs could be more toxic than larger particles."

The Guardian concludes with a suggestion from the lead study author:

Weichenthal said he avoided heavily polluted streets when walking and cycling. “At an individual level, it is always a good idea to reduce your exposure to pollutants. But the more important actions are at a regulatory level, where you can take action that reduces everyone’s exposure – that is where the real benefits come in.”

Condos on Gardiner Expressway, Toronto/ Lloyd Alter on the one day bikes are allowed there./CC BY 2.0

In Toronto where I live, the City planning department keeps approving condos that almost hang over the expressways. In many cities, crowded main streets are where the apartment buildings get approved. Now that all these studies are correlating cars with cancer, perhaps it is time to move people further away from them, or just recognize all the different ways cars are killing us and get rid of them.